The field of the invention is wheels, and the invention relates more particularly to wheels for shopping carts.
Shopping carts provide a unique challenge in that the shopping carts are becoming larger and, particularly in super stores, are often loaded with a substantial weight. While prior art wheels were satisfactory for the smaller and lighter carts, the trend toward heavier carts has resulted in unacceptable stress to the wheel bearings, often leading to bearing failure or bearing separation from the wheels. Furthermore, the tile floors commonly used in stores has changed. Whereas in the past asbestos was used as a reinforcing fiber, this is no longer the case. The resulting newer tiles are thus much softer than the old tiles.
In the past, the most common shopping cart wheel was a two part rubber wheel which had a hard rubber core which was coated with a bonding agent and a softer rubber tread was molded onto the hard rubber core. The bearings were supported by a cylindrical axial opening. With the heavier loads, the bearing tends to become loose from the hard rubber core, particularly when the cart is rolled through a turn over a rough surface such as a parking lot while the cart is carrying a heavy load. Also, the softer rubber tread often breaks away from the heavy load, resulting in a very rough ride. Furthermore, the black rubber tread can mark the floor with black streaks
Because of the shortcomings of the two-part rubber wheels, there has been a trend toward a polyurethane wheel, which has captured the majority of the present market. The polyurethane wheel has a plastic hub and a polyurethane tread is injection molded on the outer surface. Unfortunately, the polyurethane tread is relatively hard and results in a rough ride for a loaded cart, particularly on a parking lot or other roughened surface. Furthermore, because the floor tiles are now softer without the presence of asbestos, the relatively hard polyurethane tread tends to dig into the floor and damage it. Also, because the polyurethane tread is relatively hard, the shock caused by going over a rough surface is transferred to the bearings, and the bearings often are destroyed or break away from the axial opening in which they are kept. Lastly, polyurethane wheels can cause a problem with static electricity. The acrylic floor waxes commonly used today, when combined with the low humidity created by the refrigeration systems used in the store, results in a buildup of static electricity when a cart having polyurethane wheels is wheeled along a polished floor. When the shopper is wearing insulative, rubber-soled shoes, the charge passes up through the cart, into the shopper and, when the shopper touches an electrically grounded display case frame, an uncomfortable spark can result.